(1) Field of the Invention
The present invention relates to a liquid crystal display unit.
(2) Description of the Prior Art
The display system of liquid crystal display unit may roughly be divided into the following two types.
One of the display systems is such a conventional system as represented by the twist nematic display mode (hereinafter referred to as xe2x80x9cTN typexe2x80x9d), wherein a pair of transparent electrodes are opposingly arranged on substrates different from each other to operate a liquid crystal loaded between these electrodes by applying a voltage between them.
In recent years, another system replacing the above, in which the direction of the electric field formed around the liquid crystal is made almost parallel to the substrate surface (In-Plane Switching; hereinafter referred to as xe2x80x9cIPS typexe2x80x9d), has been proposed in, for example, Japanese Patent Publication Sho 63-21907, WO91/10936, Unexamined Japanese Patent Publication Hei 6-160878.
FIG. 20a shows an example of the IPS type pixel.
An array substrate la has pixel electrodes 4 and common electrodes 3 on a upper surface thereof. A semiconductor switching element 7 consisting of thin film transistor (TFT) controls the switching-on/off of the connection between a video signal line 5 and a pixel electrode 4 by a signal from the scanning signal line 6. A gate insulator film 11 prevents the short-circuiting between the common electrode 3 and the video signal line 5. The protective insulator film 12 protects the semiconductor switching element 7. A liquid crystal layer 2 is formed in the space between the array substrate 1a and counter substrate 1b. When the semiconductor switching element 7 turns on, a voltage is applied between the pixel electrodes 4 and common electrodes 3. Consequently, an electric field is produced between the adjoining pixel electrode 4 and common electrode 3 to activate the liquid crystal in the liquid crystal layer 2.
According to this system, a far wider viewing angle than that of the TN type can be obtained.
However, the IPS type display system has such problematical points as follows.
The IPS type has a problem of xe2x80x9ccoloringxe2x80x9d in which the apparent tone of color varies depending upon the direction of visual angle, that is, the color is seen blue when viewed from the angle of the longer axis of the liquid crystal molecule, and it is seen red if viewed from the direction of the shorter axis, although it actualized a wider viewing angle. This being due to the anisotropy of refractive index peculiar to the liquid crystal, it often elicits the troubles at the time of bright state display (at the time of voltage application in the normally black mode).
Another problematic point for the IPS type is that the after image is more likely to occur. This image retention is thought to be due to the occurrence of ionic polarization in liquid crystal phase, because of the asymmetric distribution of the electric field formed between the pixel electrode and common electrode.
Furthermore, a transmittance-voltage curve (hereinafter referred to as xe2x80x9cT-V curvexe2x80x9d) of the IPS type display unit is steeper than that of the conventional TN type, and consequently it is difficult to control the tone.
Besides, the IPS type color liquid crystal display unit also has a problem in that a light use efficiency is low. Generally, the color display is enabled by using red, green, or blue color filter for each pixel. As shown in FIG. 19, the transmittance of color filter depends on each color. Since a wavelength distribution of a light source is not uniform, if the same voltage is applied on each pixel under such state, luminance of respective colors will become divergent, thereby prohibiting the displaying of any achromatic color. Because of this inconvenience, the conventional filters was so designed that the voltage to be applied should be corrected according to each color, or that the transmittance of the filter at each color should coincide with each other, while retaining the applied voltage to be the same. The voltage correction requires the matching of light intensity with the darkest color. Meantime, according to the design of filter transmittance, it is required to reduce the transmittance of any one of the filters. Specifically, in the case of so-called reflection type liquid crystal display unit, the light inevitably passes twice through the color filter, and therefore it has become very difficult to set the chromatic purity and transmittance of the filter.
The object of the present invention is to solve the foregoing problems and to provide a liquid crystal display unit with fast-response, a high light use efficiency, little image retention, and a wider viewing angle.
The liquid crystal display according to the present invention has:
plural pixels each includes common electrodes, pixel electrodes and a semiconductor switching element;
scanning signal lines; video signal lines for outputting a signal to the pixel electrode;
an array substrate on which the plural pixels, the scanning signal lines, and the video signal lines are arranged;
a counter substrate opposingly arranged to the array substrate; and
a liquid crystal layer sandwiched between the array substrate and the counter substrate, wherein at least one of the electrode pairs consisting of a set of the common electrode and the pixel electrode or at least one of the electrodes has a different geometrical shape from others.
The IPS type liquid crystal display unit of the present invention, where the common electrodes and pixel electrodes are alternately arranged in a row on the same substrate, concomitantly uses the electrode pairs different in geometrical shape from other electrode pairs, or the electrodes different in geometrical shape from other electrodes.
The present invention includes a case where the electrodes or electrode pairs different in geometrical shape from each other coexist in the same pixel, and another case where the pixels different from each other in the configuration of the electrodes or electrode pairs coexist.
The geometrical shape of the electrode in the present invention includes the width and thickness of the electrode. For example, the pixel electrodes different in the width or thickness from the common electrodes are used. The present invention also includes a case where the common electrode different in the width or thickness from other common electrodes is employed, and another case where the pixel electrode different in the width or thickness from other pixel electrodes is employed.
The geometrical shape of the electrode pair in the present invention includes a gap between the both electrodes which constitute an electrode pair.
The combined use of the common electrodes and pixel electrodes of different width and thickness in the same pixel enables a formation of an electric field distribution which is symmetrical on the axis of the center line of the both electrodes, between these electrodes, and enables to suppress the occurrence of image retention.
If a region, where the geometrical shape of the electrode pairs is different from that of other electrode pairs, is locally provided in the same pixel, the plural electric field distributions, that are different in geometrical shape from each other, will be formed. Consequently, the plural are as that are different in the direction of the director of the liquid crystal molecule from each other will be formed in the pixel. Since the coloring in these areas cancels each other out, it is possible to widen the viewing angle of the display unit. Particularly, it is more effective to arrange such electrode pairs that constitute the electric fields consisting mainly of vertical components. To effectively construct the electric fields, which are vertical in direction, it is desired to make the electrode width or the gap between the common electrode and pixel electrode not larger than the gap between the array substrate and counter substrate. It is also useful to provide other electrodes on the counter substrate. Preferably, the potential of the electrodes thus newly provided shall be made equivalent to that of the common electrode.
The coexistence of the electrode pairs with different electrode gaps contributes further to the enhancement of fast response. This is because the molecules of liquid crystal corresponding to the electrode pairs with wider gap follow the liquid crystal molecules corresponding to the electrode pairs with narrower gap that respond more quickly. This also makes it possible to suppress the image retention. For instance, it can be made possible to approximate the geometrical form of the electric field distribution formed between both electrodes to symmetry by providing broader electrodes to the regions more susceptible to the influence of the potential of surrounding wiring.
A useful method to use the pixels where the electrodes or electrode pairs are different in geometrical shape from each other is to use the pixels having their own electrode configuration corresponding to respective display colors. The geometrical shape of the electric field distribution generated on the electrode pair depends on the geometrical shape of the electrode pair. Consequently, if the geometrical shape of the electrode pair is changed, the wavelength dispersion characteristic of the liquid crystal layer will also be changed. Therefore, the geometrical forms of the electrode pair such as the electrode gap, electrode width, and electrode thickness shall be set so that the wavelength, at which the light passing through the liquid crystal layer manifests its peak, can be close to the wavelength at which the color filter of respective colors R, G, and B manifests its peak transmittance. This provides highly efficient use of light as well as a high luminance. Since each pixel suited to each display color can be obtained by making the pixel of each color have its own electrode configuration, it becomes easier to set the chromatic purity and transmittance of the filter of which degree of freedom was conventionally low. A coexistence in a pixel of the electrodes or electrode pairs different in geometrical shape from each other may endow the T-V characteristic of the pixel with tonality. Making the T-V characteristic smoother contributes to a display with a higher tonality.
According to the present invention, since the spectral transmittance characteristic of liquid crystal layer can be controlled in due consideration of the wavelength distribution of light source and the wavelength dependability of transmittance of the color filter, a pixel with more excellent tone can be obtained.
As shown in FIG. 22, when curved electrodes (hereinafter referred to as xe2x80x9cangular-shaped electrodesxe2x80x9d) are used as the common electrode 3 and pixel electrode 4, the electrodes different in the angle xcex8 (hereinafter referred to as xe2x80x9cangle of bendxe2x80x9d) from each other shall be used in combination. Such angular-shaped electrodes have a geometrical shape in which a pair of linear electrode portions are linked at the end of each other. Therefore, since the electric field distributions of different geometrical shapes are formed in the areas A and B in the figure, the coloring in both areas will canceled each other. When the angle of bend is different between electrodes that constitute an electrode pair as in the present invention, the geometrical shape of distribution of the electric field formed at each region in the same area differs from each other, and therefore, such configuration of the present invention is more effective for suppressing the coloring. To prevent a decrease in an aperture ratio, that is to decrease an area of shield portions shared by black matrix, it is desirable to use the electrodes smaller in the angle of bend than that of other electrodes in the place near the video signal lines, for instance. More preferably, the difference of the angle of bend shall not be greater than 10 degrees.
In the case where the angular-shaped electrodes are to be used for the common electrode and pixel electrode, the angle of bend of the electrodes included in the pixel for red color display will preferably be larger than those of the electrodes included in the pixel for displaying other colors.
The present invention is not limited to so-called transmission-type liquid crystal display unit but also be applied to so-called reflection-type liquid crystal display unit provided with a reflection part, which is intended to reflect the incident ray from the exterior.
According to the present invention, desired electric field distribution can be produces on the electrode pair by combining a part of the electrodes or electrode pairs different in the geometrical shape (that is, electrode gap, electrode width and electrode thickness) from others depending on respective purposes such as coloring offset, larger aperture ratio, faster response. Because the minimum width and working accuracy of the pixel electrode and common electrode differs depending on the configuration of the pixel, electrode material, process conditions, etc., it is necessary to discuss the electrode configuration in due consideration of these factors. To cancel each coloring, for example, priority should be given to the setting of the angle of bend of the electrode. To have a larger aperture, the first priority should be given to the setting of such an electrode width that the finest line can be manifested in the manufacturing process of the electrode, and then setting should be made to set the electrode gap larger. For the faster response, priority should be given to such a setting of the electrode thickness that will enable the thickest film in the manufacturing process, and the electrode gap should be set narrower. However, note that the present invention renders the degree of design freedom significantly higher than conventional one.